A method of forming metal patterns on a substrate, particularly forming the patterned circuitry on high density electronic packaging, utilizes a technique termed "ablative photodecomposition." This technique, which is a method of dry imaging, involves depositing an organic polymer coating on the substrate to be patterned. The polymer coating, which is sensitive to radiation, is then patterned by irradiation typically from a laser, usually an excimer laser. A large number of photons of a particular wavelength are directed to the polymer coating in a short time. The polymer coating, which must be capable of absorbing at the laser wavelength, absorbs a significant portion of these photons. As a result, in certain types of laser ablation, many polymer chain fragments are produced in a small volume in a very short time. This causes a localized increase in pressure which cannot be sustained, and the pressure is relieved by the process of ablation, wherein fragmented polymer chains explode and escape from the coating, leaving an etched material. Thus, the material is patterned without a solvent development stage. However, the absorption efficiency of a particular ablatable polymer coating at a particular wavelength may be quite low so that considerable laser energy is required to ablate the polymer coating.
To increase the absorption efficiency of the polymer coating at the particular wavelength to which the polymer coating will be exposed, photoabsorbers have been mixed with the polymers. The photoabsorber is selected which will have an absorbance at or near the wavelength of the laser that will be employed in the ablation.
However, with conventional polymer-photoabsorber mixtures, the photo absorber is not uniformly distributed within the coating. This lack of uniformity arises in part from uneven mixing processes. In addition, over time, lack of uniform photoabsorber distribution develops from crystallization, phase separation, and migration of the photo absorber to the surface or to the interface of the polymer coating and the substrate. As a result of nonuniform distribution of the photoabsorber, increased ablation occurs in the area of the high photoabsorber concentration, leading to poor circuit pattern quality, and poor resolution. Conversely, in areas which have little or no photoabsorber, little or no ablation occurs which often leads to short circuits.
It would be desirable to have an ablatively photodecomposable composition with uniformly dispersed photoabsorber to provide evenly ablated patterns.